The present invention relates to a core for an automobile bumber.
In recent years, a bumper made of a synthetic resin has been developed in place of a bumper made of a metal according to a tendency to make an automobile lighter. Then, a bumper comprising a core made of a synthetic resin foam body and a cover made of a non-foamed synthetic resin (a block body of synthetic resin) which covers the core has been employed.
The core is an important element which influences the efficiency of an automobile bumper. Therefore, it has been required that the core is not only light but also superior in an energy absorption capacity.
Hitherto, for an above-mentioned core, there have been employed
(1) a hydraulic shock absorber, PA0 (2) a polyurethane resin foam and PA0 (3) a foam comprising polystyrene and polyethylene disclosed by Japanese Unexamined Patent Publication (Tokkyo-Kokai) No. 40136/1982.
However, (1) a hydraulic shock absorber is heavier in comparison with foam and is required a long stroke to absorb a shock energy. As a result, the free design at the front of the body of an automobile is limited. (2) A polyurethane resin foam has disadvantages in quality that an energy absorption capacity and a dimensional recovery ratio as an automobile bumper decrease because a polyurethane resin foam degrades by hydrolysis caused by water in air little by little. In order to prevent a degradation, a foam should have high bulk density (i.e. 0.1 to 0.3 g/cm.sup.3), which goes against a tendency to make an automobile lighter. (3) A foam comprising polystyrene and polyethylene is resistive against a degradation by hydrolysis and light in weight. But it is short of chemical resistance (gasoline resistance) and heat resistance (heat resistance such that it is not deformed under exposure to the scorching heat of sun, i.e. about 90.degree. C.) Accordingly, it is not suitable for a core of a bumper.
As an improvement of above-mentioned materials, a polypropylene foam is disclosed by Japanese Unexamined Patent Publication (Tokkyo-Kokai) No. 221745/1983. The polypropylene foam has a bulk density of 0.015 to 0.045 g/cm.sup.3 and of which compressive stress at 50% compression is more than 1 kg/cm.sup.2. But such compressive stress is not sufficient to absorb a shock energy on condition that a stroke is short (i.e. strain is small), and a dimensional accuracy in core product becomes loose. Accordingly, such a polypropylene foam is far from an ideal core.
As a result of our researches to remove the above disadvantages, in spite of using a conventional polypropylene resin we have eventually found a core largely improved in an energy absorption capacity of foam made of a polypropylene resin at small strain by means of defining the ratio of ethylene to propylene (% by weight) and the bulk density of foam.
Accordingly, it is an objection of the present invention to provide a core for an automobile bumper which is stable in quality, light in weight and superior in an energy absorption capacity and a dimensional stability which are required for a bumper.